The present invention relates to a method for enhanced oil recovery and, in particular, to an improved steam drive operation for recovering heavy oils.
A variety of fluent driving media (e.g. compressed air, steam, CO.sub.2, water) are presently employed for recovering heavy oil from the underground strata in which such oil is typically trapped. Of these media, steam is the most widely used. However, as the oil zone is gradually depleted, oil recovery rates decrease markedly and/or the quantity of steam required to produce a barrel of oil (the steam to oil ratio) increases markedly. In order to maintain an adequate recovery rate, the rate and amount of driving steam and thus the energy required to produce such steam must be escalated. Consequently, over time, the steam to oil ratio increases and steam drive efficiency is lowered.
In an attempt to enhance the efficiency of the operation, a number of methods introduce a chemical additive such as a surface active agent (surfactant) to the steam drive. For example, see U.S. Pat. Nos. 3,412,793 and 4,086,964. In each of these processes, a discrete slug of surfactant is injected into the steam drive with the intent to create a foam block in the depleted oil zone. The steam/surfactant foam block is then followed by additional driving steam minus surfactant. The prior art postulates that the high permeability oil depleted zone is plugged by foam and the following steam is diverted by the foam block into the low permeability oil containing zone where it drives the trapped oil toward a production well. An increased pressure gradient, effected by emplacement of the foam, theoretically enhances oil recovery and efficiency of the steam drive operation.
In other processes, particularly in a lighter oil context, surfactant is employed with a non-condensible gas or a liquid such as water to enhance oil recovery. Again, discrete slugs of surfactant are introduced into the oil resevoir. For example, in the process of chemical flooding, surfactant (plus non-condensible gas) may be introduced into the resevoir for months to mobilize the oil trapped therein. An aqueous driving fluid is then injected to follow the surfactant and drive the oil toward a production well.
It may be noted that in each of the prior methods of utilizing surfactant to enhance oil recovery, the surfactant is employed to implement a piston model of driving medium in relation to the oil to be recovered. Despite the use of surfactants in such prior art, valuable additional oil typically remains trapped in the resevoir and efficiency is often less than optimally desirable. Following a certain period of steam drive operation, the level of oil recovery may become so low compared with the steam producing energy required (e.g. the steam/oil ratio may become so great) that the operation falls below the break-even point of economic or even energy feasibility. Energy feasibility occurs below 15 barrels of water equivalent steam per barrel of oil, whereas economic break-even occurs below approximately 9 barrels of steam per barrel of oil in the current economic milieu.